The present invention refers to an absorbent wetlaid paper and a method for production thereof. Such a paper is for example used as kitchen rolls, towels, toilet paper, wiping material and in absorbent articles of different kinds, e g diapers, sanitary napkins, panty liners, incontinence guards and bed protections.
It is previously known to increase the absorption capacity of paper by mixing different kinds of highly absorbent materials into the pulp. By this it is possible to obtain an absorption capacity of paper that exceeds what can be obtained by using only conventional papermaking fibers. A problem with such an admixture in a wetlaying process is how to avoid swelling of the polymer during the papermaking process. There is for example in EP 0 408 128 disclosed the use in a wetlaying process of a polymer-modified pulp, which in its alkali state swells and absorbs liquid. The wetlaying process is performed without the presence of disturbing alkali metal ions and the polymer is used in its protonized form, at which the partly dewatered paper web is brought into contact with alkali metal ions in order to activate the polymer. This involves a complicated and sensitive process.
The object of the present invention is to provide a wetlaid paper with improved absorption properties, which can be produced in a simple way with a conventional wetlaying process without complicated additional process steps. This has according to the invention been solved by the fact that the paper contains at least 1% by weight, calculated on the dry weight of the paper, of an absorbent polymeric material having a thermo-reversible liquid uptake capacity, which has a cloud point, Cp, in water within the temperature interval 30-60xc2x0 C., preferably 35-55xc2x0 C., at which the polymer has a substantially higher liquid uptake capacity at temperatures below said cloud point as compared to at temperatures above the cloud point.
As the papermaking process at the wet end normally is run at temperatures above about 45-60xc2x0 C. the thermo-reversible polymer will be inactive during the papermaking process and be activated to its swelling and absorbent form when it is brought to contact with a liquid the temperature of which is below the cloud point, Cp, of the polymer.
A polymer having a thermo-reversible liquid uptake capacity is characterized by that the polymer at a certain temperature changes its geometrical configuration, at which the hydrophilic and hydrophobic groups of the polymers take another position, resulting in that the liquid uptake capacity of the polymer is considerably changed. Thermo-reversible polymers thus at a specific temperature has a cloud point, Cp, at which the liquid uptake capacity of the polymer is changed. The cloud point, Cp, is in the interval 30-60xc2x0 C., preferably 35-55xc2x0 C., for the polymers that have been found suitable for use in the present invention.
The paper preferably contains at least 3% by weight and preferably at least 4% by weight of said thermo-reversible polymeric material.
The absorbent thermo-reversible polymeric material is preferably in the form of fibers in order to simplify its admixture into the pulp structure. It is however also possible that the material is present in another particulate form, e.g. as flakes, powder or the like.
The absorbent thermo-reversible polymeric material can be any of the following materials: crosslinked ethyl hydroxy ethyl cellulose (EHEC), crosslinked polyvinyl methyl ether (PVME), crosslinked polymers of acrylamide, starch derivatives, polymeric tensides, polyethylene glycols and copolymers thereof.
The crosslinked polymer of acryl amide can be poly-n-isopropyl acryl amide, poly-n-n-polymethacryl amide and/or poly-n-n-diethyl acryl amide.
The absorbent thermo-reversible polymeric material is preferably at least partly based on a renewable raw material.
The invention further refers to a method of producing an absorbent wetlaid paper, by adding an absorbent thermo-reversible polymeric material of the above mentioned kind to the pulp suspension, producing a wetlaid paper web of the pulp suspension to which the thermo-reversible material has been added, and then dewatering and drying the wetlaid paper web.
The thermo-reversible polymeric material is added to the pulp suspension preferably in the form of dry fibers.
The temperature of the pulp suspension is during the wet end of the papermaking process kept above the cloud point of the thermo-reversible material.